Method and apparatus for suppressing co-channel interference

ABSTRACT

System and method for interference reduction in a broadband wireless access (BWA) network are disclosed. A mobile station performs a recursive process to update channel estimates of interfering channels on a symbol-by-symbol basis for use in canceling interference from data subcarriers received through two or more antennas.

This application is a U.S. National Stage Filing under 35 9U.S.C. 371from International Application No. PCT/RU2006/000045, filed Feb. 6, 2006and published in English as WO 2007/091908 on Aug. 16, 2007, whichapplication and publication is incorporated herein by reference in theirentireties.

TECHNICAL FIELD

Some embodiments of the present invention pertain to wirelesscommunication systems. Some embodiments of the present invention pertainto wireless networks such as broadband wireless access (BWA) networks.

BACKGROUND

A mobile unit's ability to receive and process signals from a servingbase station may be affected by interfering signals, particularlyinterfering signals from other base stations that use the same frequencysubcarriers. In the case of BWA, networks, base stations of differentBWA networks may be synchronized and may concurrently transmit downlinkframes to their associated mobile stations using the same subcarriers ofa multiplexing scheme such as orthogonal frequency division multipleaccess (OFDMA). This makes it difficult for a mobile unit operating inthe presence of interfering base stations to receive and process signalsfrom a serving base station.

Thus, there are general needs for mobile stations that can operate inthe presence of interfering base stations and methods for reducinginterference.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A illustrates a wireless network in accordance with someembodiments of the present invention;

FIG. 1B illustrates a wireless network with a multi-sector base stationin accordance with some embodiments of the present invention;

FIG. 2 is a functional block diagram of a mobile station multicarrierreceiver in accordance with some embodiments of the present invention;

FIG. 3 is a block diagram of a channel estimator in accordance with someembodiments of the present invention;

FIG. 4 illustrates a downlink frame in accordance with some embodimentsof the present invention; and

FIG. 5 is a flow chart of an interference cancellation procedure inaccordance with some embodiments of the present invention.

DETAILED DESCRIPTION

The following description and the drawings illustrate specificembodiments of the invention sufficiently to enable those skilled in theart to practice them. Other embodiments may incorporate structural,logical, electrical, process, and other changes. Examples merely typifypossible variations. Individual components and functions are optionalunless explicitly required, and the sequence of operations may vary.Portions and features of some embodiments may be included in orsubstituted for those of other embodiments. Embodiments of the inventionset forth in the claims encompass all available equivalents of thoseclaims. Embodiments of the invention may be referred to herein,individually or collectively, by the term “invention” merely forconvenience and without intending to limit the scope of this applicationto any single invention or inventive concept if more than one is in factdisclosed.

FIG. 1A illustrates a wireless network in accordance with someembodiments of the present invention. Wireless network 100 includesmobile station (MS) 102, serving base station (BS) 104 and one or moreinterfering base stations (BS) 106. Interfering base stations 106 andserving base station 104 may be neighboring base stations. Mobilestation 102 and serving base station 104 communicate over main channel105. Interfering base stations 106 may communicate with other mobilestations (not illustrated) within the same frequency spectrum used bymobile station 102 and serving base station 104. Serving base station104 and interfering base stations 106 may use the same set ofsubcarriers for their communications. Interfering signals frominterfering base stations 106 may be received by mobile station 102through interfering channels 107.

In accordance with some embodiments of the present invention, mobilestation 102 may receive signals through two or more antennas,illustrated generally as antennas 103A and 103B. In these embodiments,mobile station 102 may significantly decrease, and even substantiallycancel, the interference from one or more interfering base stations 106by applying appropriate weights to the signals received through antennas103A and 103B. In some embodiments, mobile station 102 performs arecursive filtering process to generate and update channel estimates formain channel 105 and one or more interfering channels 107 on asymbol-by-symbol basis for use in generating the weights.

In some embodiments, mobile station 102 may identify interfering basestations 106 that are considered to be significant interferers based onan identifier in a preamble symbol. In some embodiments, mobile station102 may generate the subcarrier modulation sequence used by interferingbase stations 106 identified to be significant interferers.

In some embodiments, mobile station 102 may use a recursive filteringprocess to recursively generate and update channel estimates for mainchannel 105 and interfering channels 107 on a symbol-by-symbol basis foreach pilot subcarrier using pilot subcarrier modulation sequences forboth serving base station 102 and interfering base stations 106. In someembodiments, channel estimates for main channel 105 and interferingchannels 107 may be generated for each of antennas 103A and 103B. Insome embodiments, the recursive filtering process may also use thesignals received on the pilot subcarriers through each of antennas 103Aand 103B to recursively generate and update the channel estimates foreach data symbol of an OFDMA frame.

In some embodiments, mobile station 102 may calculate an interferencecorrelation matrix on a symbol-by-symbol basis for each pilot subcarrierfrom the channel estimates for main channel 105 and the channelestimates for interfering channels 107. In some embodiments, mobilestation 102 may also calculate weights on a symbol-by-symbol basis foreach pilot subcarrier for each of antennas 103A and 103B based on theinterference correlation matrix and the channel estimate for mainchannel 105. In some embodiments, mobile station 102 may interpolate theweights for each pilot subcarrier for each of antennas 103A and 103B togenerate weights for each data subcarrier for each of antennas 103A and103B. These embodiments are described in more detail below.

FIG. 1B illustrates a wireless network with a multi-sector base stationin accordance with some embodiments of the present invention. Wirelessnetwork 150 includes multi-sector base station 118 and a plurality ofmobile stations, illustrated generally as mobile stations 108, 110 and152. Multi-sector base station 118 provides wireless networkcommunications within a plurality of sectors, such as sectors 112, 114and 116, using the same frequency spectrum, and in some embodiments,using the same set of subcarriers. In these embodiments, multi-sectorbase station 118 communicates with mobile station 152 over main channel115 in sector 112; multi-sector base station 118 communicates withmobile station 108 in sector 116; and multi-sector base station 118communicates with mobile station 110 in sector 114.

In some of these embodiments, preamble symbols of frames transmitted tomobile stations of different sectors may use different sets ofsubcarriers, while the data portions of the frames may use the samesubcarriers. In some multi-sector embodiments, multi-sector base station118 may use one of three orthogonal sets of subcarriers for transmittingthe preamble symbol within each sector. In these embodiments, a basestation may use every third subcarrier for transmitting a preamblesymbol in a particular sector, although the scope of the invention isnot limited in this respect.

In accordance with some embodiments of the present invention mobilestation 152 may significantly decrease, and even substantially cancel,the interference from the communications within other sectors byapplying appropriate weights to the signals received through antennas153A and 153B. In some embodiments, mobile station 152 performs arecursive filtering process to estimate and/or update channel estimatesfor main channel 115 and one or more interfering channels 117 on asymbol-by-symbol basis for use in generating the weights. Theseembodiments are discussed in more detail below.

FIG. 2 is a functional block diagram of a mobile station multicarrierreceiver in accordance with some embodiments of the present invention.Mobile station multicarrier receiver 200 may be suitable for use as areceiver portion of mobile station 102 (FIG. 1A) and/or a receiverportion of mobile station 152 (FIG. 1B), although other receiverconfigurations may also be suitable.

Mobile station multicarrier receiver 200 may include two or moreantennas, such as antennas 203A and 203B, to receive signals. Mobilestation multicarrier receiver 200 may also include radio-frequency (RF)circuitry 204 to downconvert and digitize the received signals. RFcircuitry 204 may generate digital time-domain signals 205A from antenna203A and may generate digital time-domain signals 205B from antenna203B. Antennas 203A and 203B may correspond respectively to antennas103A and 103B (FIG. 1A) of mobile station 102 (FIG. 1A) and maycorrespond respectively to antennas 153A and 153B (FIG. 1B) of mobilestation 152 (FIG. 1B).

Mobile station multicarrier receiver 200 may also include Fouriertransform circuitry 206 to perform a Fourier transform on digitaltime-domain signals 205A to generate frequency-domain (FD) signals 207Aand to perform a Fourier transform on digital time-domain signals 205Bto generate frequency-domain signals 207B. In some embodiments, Fouriertransform circuitry 206 may provide a set of frequency-domain signalsfor each antenna. In some embodiments, for each antenna, Fouriertransform circuitry 206 may provide a frequency-domain signal for eachsubcarrier of a received multicarrier communication signal. In someembodiments, Fourier transform circuitry 206 may perform a discreteFourier transform (DFT), such as a fast Fourier transform, although thescope of the invention is not limited in this respect.

Mobile station multicarrier receiver 200 may also include channelestimator 220 to generate data-subcarrier weights 221 for each datasubcarrier based on frequency-domain signals 207A and 207B. Mobilestation multicarrier receiver 200 may also include combiner 222 tocombine and weight corresponding subcarriers of frequency-domain signals207A and 207B to generate output frequency-domain signals 223 for eachsubcarrier. In accordance with some embodiments, the application ofdata-subcarrier weights 221 by combiner 222 may suppress mostsignificant interferers, particularly the transmissions from interferingbase stations or sectors discussed above. The generation and applicationof weights by channel estimator 220 is discussed in more detail below.In some embodiments, output frequency-domain signals 223 may comprise asub-symbol for each data subcarrier. In these embodiments, combiner 222may generate a single frequency-domain signal for each data subcarrierfrom frequency-domain signals 207A and 207B associated with antennas203A and 203B, respectively.

Mobile station multicarrier receiver 200 may also include decoder 208 todecode output frequency-domain signals 223 and generate decodedsubsymbols 209. In some embodiments, subsymbols 209 may comprise one ormore quadrature-amplitude modulation (QAM) symbols for each subcarrier.

Mobile station multicarrier receiver 200 may also include demapper 210to demap decoded subsymbols 209 and to generate one or more bits 211 foreach data subcarrier. In some embodiments, demapper 210 may be a QAMdemapper to demap the QAM symbols based on a subcarrier modulationlevel, although the scope of the invention is not limited in thisrespect.

Mobile station multicarrier receiver 200 may also include deinterleaver212 to perform a deinterleaving operation on bits 211 to generatedeinterleaved bits 213. In some embodiments, deinterleaver 212 mayperform a block deinterleaving operation on blocks of bits, although thescope of the invention is not limited in this respect.

Mobile station multicarrier receiver 200 may also includeerror-correction decoder 214 to perform an error-correction decodingoperation on bits 213 to generate decoded bit stream 215. In someembodiments, error-correction decoder 214 may perform a forward-errorcorrection (FEC) decoding operation on bits 213, although the scope ofthe invention is not limited in this respect. In some embodiments,error-correction decoder 214 may perform a convolutional decodingoperation on bits 213, although the scope of the invention is notlimited in this respect.

Although mobile station multicarrier receiver 200 is illustrated ashaving several separate functional elements, one or more of thefunctional elements may be combined and may be implemented bycombinations of software-configured elements, such as processingelements including digital signal processors (DSPs), and/or otherhardware elements. For example, some elements may comprise one or moremicroprocessors, DSPs, application specific integrated circuits (ASICs),and combinations of various hardware and logic circuitry for performingat least the functions described herein. In some embodiments, thefunctional elements of mobile station multicarrier receiver 200 mayrefer to one or more processes operating on one or more processingelements.

In some embodiments, mobile station multicarrier receiver 200 may bepart of a wireless communication device that may transmit and receiveorthogonal frequency division multiplexed (OFDM) communication signalsover a multicarrier communication channel. The multicarriercommunication channel may be within a predetermined frequency spectrumand may comprise a plurality of orthogonal subcarriers. In someembodiments, the orthogonal subcarriers may be closely spaced OFDMsubcarriers. Each subcarrier of the OFDM signals may have a null atsubstantially the center frequency of the other subcarriers and/or eachsubcarrier may have an integer number of cycles within a symbol period,although the scope of the invention is not limited in this respect. Insome other embodiments, mobile station multicarrier receiver 200 maycommunicate using spread-spectrum signals. In some embodiments, thefrequency spectrums for the multicarrier communication signals maycomprise frequencies between 2 and 11 GHz, although the scope of theinvention is not limited in this respect.

In some embodiments, mobile station multicarrier receiver 200 may bepart of a broadband communication station that may operate within abroadband wireless access (BWA) network, such as a WorldwideInteroperability for Microwave Access (WiMax) network, although thescope of the invention is not limited in this respect. In someembodiments, mobile station multicarrier receiver 200 may communicate inaccordance with the IEEE 802.16-2004 and/or IEEE 802.16(e) standards forwireless metropolitan area networks (WMANs) including variations andevolutions thereof, although the scope of the invention is not limitedin this respect as they may also be suitable to transmit and/or receivecommunications in accordance with other techniques and standards. Formore information with respect to the IEEE 802.16 standards, please referto “IEEE Standards for Information Technology—Telecommunications andInformation Exchange between Systems”—Metropolitan AreaNetworks—Specific Requirements—Part 16: “Air Interface for FixedBroadband Wireless Access Systems,” May 2005 and relatedamendments/versions.

In some embodiments, mobile station multicarrier receiver 200 may bepart of a portable wireless communication device, such as personaldigital assistant (PDA), a laptop or portable computer with wirelesscommunication capability, a web tablet, a wireless telephone, a wirelessheadset, a pager, an instant messaging device, a digital camera, anaccess point, a television, medical devices (e.g., a heart rate monitor,a blood pressure monitor, etc.), or other device that may receive and/ortransmit information wirelessly.

Antennas 203A and 203B may comprise two or more directional oromnidirectional antennas, including, for example, dipole antennas,monopole antennas, patch antennas, loop antennas, microstrip antennas orother types of antennas suitable for transmission of RF signals. In someembodiments, instead of two or more antennas, a single antenna withmultiple apertures may be used. In these embodiments, each aperture maybe considered a separate antenna. In some embodiments, antennas 203A and203B may be effectively separated to take advantage of spatial diversityand the different channel characteristics that may result between eachof antennas 203A and 203B and the base station. In some embodiments,antennas 203A and 203B may be separated by up to 1/10 of a wavelength ormore, although the scope of the invention is not limited in thisrespect.

FIG. 3 is a block diagram of a channel estimator in accordance with someembodiments of the present invention. Channel estimator 300 may besuitable for use as channel estimator 220 (FIG. 2), although otherchannel estimator configurations may also be suitable. Channel estimator300 generates data-subcarrier weights 221 from preamble symbol 322 of areceived downlink frame and pilot subcarriers 326 of subsequentlyreceived data symbols of the downlink frame. Data-subcarrier weights 221may be applied by combiner 222 for combining and weighing datasubcarriers 328 of the subsequently received data symbols. In accordancewith some embodiments, channel estimator 300 comprises preamble symbolprocessing circuitry 302, filter initialization circuitry 304,recursive/Kalman filter 306, weights calculation circuitry 308 andweights interpolation circuitry 310. As illustrated in FIG. 3, receivedsignals 207A and 207B may be frequency domain (FD) signals that includepreamble symbol 322 and data symbols 324. Data symbols 324 may includepilot subcarriers 326 and data subcarriers 328.

Preamble symbol processing circuitry 302 may identify one or moreinterfering base stations, such as interfering base stations 106 (FIG.1A), from preamble symbol 322 and may extract seeds 303 from thepreamble identifiers for use by filter initialization circuitry 304.Preamble symbol processing circuitry 302 may also generate initialchannel estimate 333 for main channel 105 from preamble symbol 322. Insome multi-sector embodiments, preamble symbol processing circuitry 302may also generate initial channel estimate 343 for one or moreinterfering channels 115 (FIG. 1B) from preamble symbol 322 because thepreamble symbols from different sectors may be transmitted bymulti-sector base station 118 (FIG. 1B) on different subcarriers.

In some embodiments, preamble symbol 322 may be received through two ormore antennas and preamble symbol processing circuitry 302 may combine(e.g., average or sum) the frequency-domain signals associated with eachof the antennas. In some embodiments, preamble symbol processingcircuitry 302 may perform frequency-domain matched filtering to separatepreamble symbols transmitted by interfering base station 106 (FIG. 1A)from the preamble symbol transmitted by serving base station 104 (FIG.1A), although the scope of the invention is not limited in this respect.

Filter initialization circuitry 304 may generate an initial state vector305 for recursive filter 306. Filter initialization circuitry 304 mayalso generate pilot subcarrier modulation sequences 335 for the main andthe interfering channels from extracted seeds 303. In some embodimentsfilter initialization circuitry 304 may select the orders (i.e.,dimensions) for the recursive filter 306 based on a number ofinterfering base stations 106 (FIG. 1A) identified by preamble symbolprocessing circuitry 302. In some embodiments, filter initializationcircuitry 304 may adaptively update the orders of recursive filter 306when the number of interfering base stations 106 changes, although thescope of the invention is not limited in this respect. In someembodiments, the orders of recursive filter 306 may be adaptivelyredistributed based on the number, strength and/or importance of signalsfrom interfering base stations 106 (FIG. 1A). For example, when a forthorder recursive filter is selected, two orders may be allocated to mainchannel 105 (FIG. 1A), and one order may be allocated to eachinterfering channel 107 (FIG. 1A).

Recursive filter 306 may generate channel estimates 307 for the mainchannel and the interfering channels by performing a symbol-by-symbolrecursive process using initial state vector 305, pilot subcarriermodulation sequences 335, and pilot subcarriers 326 of each data symbolreceived through each antenna. In some embodiments, recursive filter 306may be a Kalman filter, although the scope of the invention is notlimited in this respect. Some embodiments of the symbol-by-symbolrecursive process are described in more detail below.

Weights calculation circuitry 308 may generate pilot-subcarrier weights309 for each pilot subcarrier for each of antennas 203A and 203B (FIG.2) based on channel estimates 307. Weights interpolation circuitry 310may interpolate pilot-subcarrier weights 309 to generate data-subcarrierweights 221 (FIG. 2) for each data subcarrier 328 for each antenna. Insome embodiments, weights calculation circuitry 308 may perform a linearinterpolation to generate pilot-subcarrier weights 309. In someembodiments, weights calculation circuitry 308 may perform an optimalweight calculation. In these embodiments, pilot-subcarrier weights 309may be calculated by weight calculation circuitry 308 to suppress thesignals from the most significant of interfering base stations 106 (FIG.1A). In some embodiments, weight calculation circuitry 308 may apply azero-forcing (ZF) algorithm to calculate pilot-subcarrier weights 309,while in other embodiments, weight calculation circuitry 308 may apply aminimum mean squared error (MMSE) algorithm to calculatepilot-subcarrier weights 309, although the scope of the invention is notlimited in these respects.

Although channel estimator 300 is illustrated as having several separatefunctional elements, one or more of the functional elements may becombined and may be implemented by combinations of software-configuredelements, such as processing elements including digital signalprocessors (DSPs), and/or other hardware elements. For example, someelements may comprise one or more microprocessors, DSPs, applicationspecific integrated circuits (ASICs), and combinations of varioushardware and logic circuitry for performing at least the functionsdescribed herein. In some embodiments, the functional elements ofchannel estimator 300 may refer to one or more processes operating onone or more processing elements.

FIG. 4 illustrates a downlink frame in accordance with some embodimentsof the present invention. Downlink frame 400 comprises preamble symbol402 and one or more data symbols 404. Preamble symbol 402 may correspondto preamble symbol 322 (FIG. 3) and may include, among other things, apreamble identifier from each transmitting base station and anassociated seed for generating the subcarrier modulation sequence forthe associated base station. Data symbols 404 may comprise pilotsubcarriers corresponding to pilot subcarriers 326 (FIG. 3) and datasubcarriers corresponding to data subcarriers 328 (FIG. 3). In someembodiments, downlink frame 400 may be an OFDMA frame, and preamblesymbol 402 and data symbols 404 may comprise OFDMA symbols. Althoughframe 400 is described as a downlink frame transmitted from a basestation to a mobile station, the scope of the invention is not limitedin this respect and may equally apply to uplink transmissions.

Referring to FIGS. 1A, 1B, 2, 3 and 4 together, some embodiments of thepresent invention may provide for interference mitigation in broadbandwireless access systems, such as systems operating in accordance withthe IEEE 802.16e standard. In some conventional broadband wirelessaccess systems, one of the major causes of downlink performancedegradation is a result of interference from neighboring base stations.In accordance with some embodiments of the present invention, mobilestation 102 may significantly decrease and possibly even completelycancel the interference from neighboring base stations by applyingweights to the signals received on different antennas 103A and 103B. Insome embodiments, the weight-application technique discussed above maycomprise optimal combining or adaptive interference cancellation. Insome embodiments, zero forcing or minimum mean square error approachesmay be used to calculate the antenna weights at each of the subcarriers,although the scope of the invention is not limited in this respect.

Some conventional interference cancellation algorithms require use ofthe interference correlation matrix at each subcarrier. Directestimation of the interference correlation matrix at each subcarrier maybe a difficult and computationally complex task. In accordance with someembodiments of the present invention, interference cancellation may bebased on channel estimates of the interfering channels (e.g., channels107 or channels 117) rather than a direct estimation of the interferencecorrelation matrix.

Because neighboring base stations may operate synchronously, the pilotsubcarriers of serving base station 104 may be affected by the pilotsubcarriers of interfering base stations 106 because the pilotsubcarriers of the neighboring base stations use the same frequencysubcarriers. Each base station however, may modulate its pilotsubcarriers with different pilot subcarrier modulation sequences. Insome embodiments, the pilot subcarrier modulation sequences may comprisepseudo random binary sequences (PRBSs), although the scope of theinvention is not limited in this respect. As discussed above,multi-sector base station 118 may modulate the pilot subcarriers used ineach sector with different pilot subcarrier modulation sequences. Insome embodiments, information about each pilot subcarrier modulationsequences may be determined from a special identifier which may beincluded in a frame header and/or from the preamble symbol using thepreamble processing discussed above. For example, a preamble symbol ofeach base station may include a preamble identifier (ID) from which aseed may be extracted. The seed may be used to seed an algorithm togenerate the particular subcarrier modulation sequence used by the basestation that transmitted the preamble symbol. The pilot subcarriermodulation sequence of serving base station 104 may be used for channelestimation of main channel 105 by mobile station 102. In accordance withsome embodiments, the pilot subcarrier modulation sequences ofinterfering base stations 106 may be used by channel estimator 220 tocalculate an interference correlation matrix through estimating theinterfering channels for the purpose of further interferencecancellation.

In accordance with some embodiments, the received signal at a particularpilot subcarrier may be expressed as:

$\begin{matrix}{{x_{r}(k)} = {{{H_{m}(k)}{p_{m}(k)}} + {\sum\limits_{i = 1}^{N\mspace{14mu}{int}}{{H_{i}(k)}{p_{i}(k)}}} + {n(k)}}} & (1)\end{matrix}$where x_(r)(k) is a received signal vector of size 1×N_(int) at allreceiving antennas at the k^(th) symbol, p_(m)(k) represents the pilotvalues for serving base station 102, p_(i)(k) represents the pilotvalues for interfering base stations 106, and n(k) is residual noisewhich may be modeled as additive white Gaussian noise (AWGN). Theresidual noise may include background receiver noise and weakinterferers. In Equation (1), H_(m) is a vector representing the channeltransfer function for main channel 105, and H_(i) (i=1 . . . N_(int)) isa vector representing the channel transfer functions for interferingchannels 107 from interfering base stations 106 to mobile station 102.N_(int) represents the number of interfering base stations 106 that aredetermined to be significant interferers by preamble symbol processingcircuitry 302.

In accordance with some embodiments, a state-space dynamic model of thechannel transfer function may be used and recursive filtering, such asKalman filtering, may be applied to estimate main channel 105 andinterfering channels 107 simultaneously. The recursive filteringperformed by recursive filter 306 may perform matrix processingoperations at each step. These matrix processing operations aregenerally more computationally intensive for higher filter orders.Therefore, the orders of the filter may be selected based on a minimumnumber of interfering base stations. In accordance with someembodiments, the orders of recursive filter 306 may be adaptivelyredistributed between main and interfering base stations in accordanceto their relative importance and strength. For example, for 1×2 systemwith two interfering base stations, the channel state-space model may berepresented by the following equation:H _(m)(k+1)=H _(m)(k)+{dot over (H)} _(m)(k)+w _(m)(k)H _(i1)(k+1)=H _(i1)(k)+w _(i1)(k)H _(i2)(k+1)=H _(i2)(k)+w _(i2)(k)   (2)

where H_(m)(k) represents the channel transfer function representing thechannel estimate for the main channel for the k^(th) symbol, H_(i1)(k)represents the channel estimate for the first interfering channel forthe k^(th) symbol, and H_(i2)(k) represents the channel estimate for thesecond interfering channel for the k^(th) symbol. In Equation (2) w mayrepresent the model noise, which may describe stochastic channelbehaviour and may represent a discrepancy between the model and realchannel dynamics. {dot over (H)}_(m), read Hdot(k), represents thederivative of the channel transfer function and may be calculated fromthe expression {dot over (H)}_(m)(k)=H(k)−H(k−1). In accordance withsome embodiments, first and even higher-order derivatives may be used todescribe smooth or rapidly changing channels.

In some embodiments, main channel 105 between base station 104 andmobile station 102 may be considered non-stationary because mobilestation may be moving. In these embodiments, a higher-order model (e.g.,higher than a first-order model) may be used for the state-space modelof the channel. However, from an implementation point of view, the orderof recursive filter 306, which may comprise the orders of the channelmodels of the main and interfering channels, may be a pre-determinedvalue, although the scope of the invention is not limited in thisrespect.

In accordance with some embodiments, a generalized channel vector H foreach antenna may be described by the following equation:

$\begin{matrix}{{H(k)} = \begin{bmatrix}{H_{m}(k)} \\{{\overset{.}{H}}_{m}(k)} \\{H_{i\; 1}(k)} \\{H_{i\; 2}(k)}\end{bmatrix}} & (3)\end{matrix}$

From Equation (2) and (3), the channel state-space model may berewritten in matrix form using a transition matrix F:

$\begin{matrix}{{{H\left( {k + 1} \right)} = {{F \star {H(k)}} + {w(k)}}}{{{or}\mspace{14mu}{{equivalent}\begin{bmatrix}{H_{m}\left( {k + 1} \right)} \\{{\overset{.}{H}}_{m}\left( {k + 1} \right)} \\{H_{i\; 1}\left( {k + 1} \right)} \\{H_{i\; 2}\left( {k + 1} \right)}\end{bmatrix}}} = {{\begin{bmatrix}1 & 0 & 0 & 0 \\0 & 1 & 0 & 0 \\0 & 0 & 1 & 0 \\0 & 0 & 0 & 1\end{bmatrix} \star \begin{bmatrix}{H_{m}(k)} \\{{\overset{.}{H}}_{m}(k)} \\{H_{i\; 1}(k)} \\{H_{i\; 2}(k)}\end{bmatrix}} + {\begin{bmatrix}{w_{m}(k)} \\0 \\{w_{i\; 1}(k)} \\{w_{i\; 2}(k)}\end{bmatrix}.}}}} & (4)\end{matrix}$

In some embodiments, recursive filter 306 may use two orders (i.e., twodimensional vectors H_(m) and {dot over (H)}_(m)) to track main channeland two orders to track channel of each interfering channel for a totalof four orders. This may be viewed as a forth-order system. In someembodiments, when only one interfering base station is present, theorders of recursive filter 306 may be reallocated accordingly by filterinitialization circuitry 304 to better track the more ‘significant’interfering channel, although the scope of the invention is not limitedin this respect.

In accordance with some embodiments, Equation (1) may represent theobservation model and Equation (2) may represent the state-space modelthat may be used to construct recursive filter 306 for estimating thetransfer functions of the main and interfering channels.

In some embodiments, symbol-by-symbol recursive filtering performed byrecursive filter 306 may include the operations represented by thefollowing equations to update the channel estimates for the main andinterfering channels:K(k)=V _(H)(k|k−1)p(k)[p(k)^(T) V _(H)(k|k−1)p(k)+V _(n)]⁻¹   (5)

which represents a gain update,H(k+1)=FH(k)+K(k)└x(j)−p(k)^(T) FH(k)┘  (6)

which represents a channel estimate update,V _(H)(k)=[I−H(k)K(j)]V _(H)(k|k−1)   (7)

which represents an a-posteriori estimate covariance matrix for thecurrent symbol, andV _(H)(k+1|k)=FV _(H)(k)F ^(T) +V _(w)   (8)

which represents a prediction of an a-priori estimate of the covariancematrix for next symbol.

The initial conditions for recursive filter 306 may be set based oninitial channel estimate H(0) that may be generated from preamble symbol402 or alternatively may be taken from the previous frame. In the aboveequations, V_(H) represents a priori estimate covariance, V_(n)represents an observation additive noise covariance, V_(w) representssignal model noise covariance, and F is a transition matrix. In someembodiments, V_(H) may depend on the accuracy of the initial conditions.When the initial conditions are generated from the preamble symbol,V_(H) may be set to a pre-defined value corresponding to the channelestimate variance. When the initial conditions are inherited from aprevious frame for which recursive filtering was performed, the value ofV_(H) at the end of the previous frame may be used. In some embodiments,some correction may be applied. In some embodiments which utilize tworeceive antennas V_(n) may be a 2×2 diagonal matrix with noisedispersion as the diagonal elements of the matrix. The noise dispersionmay be determined from either known or measured input receiver noise.

After recursive filter 306 generates channel transfer functions 307 formain channel 105 and interfering channels 107 for the most significantinterferers using Equations (5) thorough (8), weights calculationcircuitry 308 may calculate the interference correlation matrix [R_(in)]from the channel transfer functions for interfering channels 107 and mayapply an algorithm, such as a zero-forcing algorithm or MMSE algorithm,to generate weights for each pilot subcarrier for each antenna. In someembodiments, weights calculation circuitry 308 may apply the followingequations to calculate the interference correlation matrix [R_(in)] andthe weights [w] for each pilot subcarrier:

$\begin{matrix}{R_{in} = {{\sum\limits_{i = 1}^{N\mspace{14mu}{int}}{H_{i}^{H}H_{i}}} + {I\;\sigma_{n}^{2}}}} & (9) \\{w = {\left( {H_{m}^{H}R_{in}^{- 1}H_{m}} \right)^{- 1}H_{m}^{H}R_{in}^{- 1}}} & (10)\end{matrix}$

In Equation (10), w represents a weight vector. Weights interpolationcircuitry 310 may perform a linear interpolation on the weightsgenerated by weights calculation circuitry 308 to generate weights foreach data subcarrier for each antenna of the current data symbol. Theweights generated by weights calculation circuitry 308 may be in theform of a weight vector. Combiner 222 may apply the weight vectorgenerated by weights calculation circuitry 308 to the received signal ata particular subcarrier for the current data symbol to mitigateinterference.

Because frequency domain signals 207A and 207B (FIG. 3) are received byseparate antennas, slight differences between frequency domain signals207A and 207B may result because of the differing channelcharacteristics between antenna 103A and base stations 104, 106, andantenna 103B and base stations 104, 106. Recursive filter 306 exploitsthese slight differences between frequency domain signals 207A and 207Ballowing combiner 222 to perform an optimal combining technique, such asthe ZF and MMSE optimal combining techniques discussed above.

In some embodiments, channel estimator 220 may take advantage of thedifference in phase shift between the signals received by antennas 203Aand 203B from serving base station 102 and the signals received byantennas 203A and 203B from interfering base stations 106. For example,when antennas 203A and 203B are omnidirectional antennas, the phaseshift of the signals received from serving base station 102 betweenantennas 203A and 203B will be different than the phase shift of thesignals received from one of the interfering base station 106. This isbecause the direction to serving base station 102 may be different thanthe direction to an interfering base station. The different directionsresult in different phase shifts of signals received by antennas 203Aand 203B. In other words, signals received from serving base station 102will have one phase shift, and the signals from each interfering basestation 106 will have different phase shifts. Based on these differentphase shifts, as well as other parameters described above, channelestimator 220 may generate weights that may cancel the interferingsignals.

In some embodiments in which two receive antennas are used (i.e.,antennas 203A and 203B), channel estimator 220 may generate weights toalmost completely cancel interference from one interfering base stationand at least partially cancel interference from other interfering basestations arriving from different directions. In some embodiments inwhich three receive antennas are used, channel estimator 220 maygenerate weights to completely cancel interference from two interferingbase stations arriving from different directions. In some embodiments,when an adaptive array of receive antennas is used with N elements,channel estimator 220 may generate weights to cancel interference fromN-1 interfering base stations arriving from different directions. Inthese embodiments, the number of elements N may range from as little astwo to as great a ten or more.

FIG. 5 is a flow chart of an interference cancellation procedure inaccordance with some embodiments of the present invention. Interferencecancellation procedure 500 may generate and apply weights to signalsreceived through two or more antennas of a mobile station, such asmobile station 102 (FIG. 1A), to significantly decrease and evensubstantially cancel the interference from one or more interfering basestations, such as interfering base stations 106 (FIG. 1A). In someembodiments, interference cancellation procedure 500 may be performed bya channel estimator, such as channel estimator 220 (FIG. 2), to generatethe weights, and a combiner, such as combiner 222 (FIG. 2), to apply theweights, although other devices may be used to perform procedure 500.

In operation 502, interfering base stations are identified from areceived preamble symbol. In some embodiments, the preamble identifiersfor the interfering base stations are determined from the preamblesymbol. In some embodiments, the interfering base stations areidentified from the preamble symbol and the most significant interferingbase stations are identified. In some embodiments, the two mostsignificant interfering base stations are identified, although the scopeof the invention is not limited in this respect. In some embodiments,the preamble identifier for the serving base station may also bedetermined. In some embodiments, operation 502 may also includedetermining an initial channel estimate for the main channel from thepreamble symbol. In some multi-sector embodiments, operation 502 mayalso include determining an initial channel estimate from interferingchannels of a multi-sector base station from the preamble symbol. Insome embodiments, operation 502 may be performed by preamble symbolprocessing circuitry 302 (FIG. 3), although the scope of the inventionis not limited in this respect.

Operation 504 comprises extracting seeds from the preamble identifiersfor the serving base station and the interfering base stations andgenerating subcarrier modulation sequences from the extracted seeds. Insome embodiments, operation 504 may be performed by preamble symbolprocessing circuitry 302 (FIG. 3) and/or filter initialization circuitry304 (FIG. 3), although the scope of the invention is not limited in thisrespect.

Operation 506 comprises initializing a recursive filter. In someembodiments, operation 506 includes initializing the recursive filterwith the initial channel estimates in accordance with the state-spacemodel discussed above. In some embodiments, operation 506 may beperformed by filter initialization circuitry 304 (FIG. 3), although thescope of the invention is not limited in this respect.

Operation 508 comprises performing recursive filtering to generate andupdate the channel estimates for the main channel and the interferingchannels. In some embodiments, recursive filter 306 (FIG. 3) may performoperation 508 to recursively generate channel estimates 307 (FIG. 3) formain channel 105 (FIG. 1A) and interfering channels 107 (FIG. 2) foreach pilot subcarrier. In some embodiments, operation 508 may use pilotsubcarrier modulation sequences for both serving base station 102 (FIG.1A) and interfering base stations 106 (FIG. 1A) as well as the signalsreceived on pilot subcarriers 326 (FIG. 3) through each of the antennas.In some embodiments, recursive filter 306 (FIG. 3) may perform operation508 using Equations (5) through (8), although the scope of the inventionis not limited in this respect.

Operation 510 comprises generating weights for the pilot subcarriers. Insome embodiments, operation 510 may comprise calculating an interferencecorrelation matrix for each pilot subcarrier from the channel estimatesfor interfering channels 107 (FIG. 1A) using Equation (9). Operation 510may also comprise calculating pilot-subcarrier weights 309 (FIG. 3) foreach pilot subcarrier for each of antennas 203A and 203B (FIG. 2) basedon the interference correlation matrix and the channel estimate for mainchannel 107 (FIG. 1A) using equation (10). In some embodiments,operation 510 may be performed by weights calculation circuitry 308(FIG. 3), although the scope of the invention is not limited in thisrespect.

In operation 512, the weights for data subcarriers may be generated byinterpolating the weights generated from the pilot subcarriers inoperation 510. In some embodiments, operation 512 generatesdata-subcarrier weights 221 (FIG. 2) for each data subcarrier for eachof antennas 203A and 203B (FIG. 2). In some embodiments, operation 512may be performed by weights interpolation circuitry 310 (FIG. 3),although the scope of the invention is not limited in this respect.

In operation 514, the weights for the data subcarriers generated inoperation 512 may be applied to the signals received through two or moreantennas to suppress the interfering signals. In some embodiments,operation 514 may be performed by combiner 222 (FIG. 2), although thescope of the invention is not limited in this respect.

In some embodiments, operations 508, 510, 512 and 514 may be performedon a symbol-by-symbol basis for each data symbol of a received OFDMAframe, although the scope of the invention is not limited in thisrespect. In some embodiments, operations 502, 504, 506, 508, 510, 512and 514 may be performed on a frame-by-frame basis for each OFDMA framereceived by a mobile station. Although the individual operations ofprocedure 500 are illustrated and described as separate operations, oneor more of the individual operations may be performed concurrently, andnothing requires that the operations be performed in the orderillustrated.

Unless specifically stated otherwise, terms such as processing,computing, calculating, determining, displaying, or the like, may referto an action and/or process of one or more processing or computingsystems or similar devices that may manipulate and transform datarepresented as physical (e.g., electronic) quantities within aprocessing system's registers and memory into other data similarlyrepresented as physical quantities within the processing system'sregisters or memories, or other such information storage, transmissionor display devices. Furthermore, as used herein, a computing deviceincludes one or more processing elements coupled with computer-readablememory that may be volatile or non-volatile memory or a combinationthereof.

Embodiments of the invention may be implemented in one or a combinationof hardware, firmware and software. Embodiments of the invention mayalso be implemented as instructions stored on a machine-readable medium,which may be read and executed by at least one processor to perform theoperations described herein. A machine-readable medium may include anymechanism for storing or transmitting information in a form readable bya machine (e.g., a computer). For example, a machine-readable medium mayinclude read-only memory (ROM), random-access memory (RAM), magneticdisk storage media, optical storage media, flash-memory devices,electrical, optical, acoustical or other form of propagated signals(e.g., carrier waves, infrared signals, digital signals, etc.), andothers.

The Abstract is provided to comply with 37 C.F.R. Section 1.72(b)requiring an abstract that will allow the reader to ascertain the natureand gist of the technical disclosure. It is submitted with theunderstanding that it will not be used to limit or interpret the scopeor meaning of the claims.

In the foregoing detailed description, various features are occasionallygrouped together in a single embodiment for the purpose of streamliningthe disclosure. This method of disclosure is not to be interpreted asreflecting an intention that the claimed embodiments of the subjectmatter require more features than are expressly recited in each claim.Rather, as the following claims reflect, invention may lie in less thanall features of a single disclosed embodiment. Thus the following claimsare hereby incorporated into the detailed description, with each claimstanding on its own as a separate preferred embodiment.

What is claimed is:
 1. A mobile station that operates in presence of oneor more interfering base stations comprising: a preamble symbolprocessing circuitry configured to identify one or more interfering basestations from a received preamble symbol; a recursive filter circuitryconfigured to perform a recursive filtering process using pilotsubcarrier modulation sequences on pilot subcarriers of orthogonalfrequency division multiple access (OFDMA) data symbols received throughtwo or more antennas to update channel estimates on a symbol-by-symbolbasis for a main channel associated with a serving base station and oneor more interfering channels associated with the one or more interferingbase stations for use in mitigating interference from the interferingbase stations, wherein the recursive filter updates the channelestimates for the main channel and the one or more interfering channelson the symbol-by-symbol basis based on a state-space dynamic model of achannel transfer function for the main and interfering channels usingsignals received on the pilot subcarriers and the subcarrier modulationsequences for the main and interfering channels; and a filterinitialization circuitry configured to generate the pilot subcarriermodulation sequences for the main channel and the one or moreinterfering channels based on seeds extracted from preamble identifierstransmitted by the serving base station and the one or more interferingbase stations respectively, the recursive filter being initialized withchannel estimates generated from the pilot subcarrier initialized withchannel estimates generated from the pilot subcarrier modulationsequences, wherein the serving base station and the one or moreinterfering base stations use different pilot subcarrier modulationsequences for modulation of their pilot subcarriers.
 2. The mobilestation of claim 1 further comprising weights calculation andinterpolation circuitry to generate weights fur subsequent applicationto each of a plurality of data subcarriers of the data symbols receivedthrough the two or more antennas, wherein the weights calculation andinterpolation circuitry calculates an interference correlation matrixfrom the channel estimates for the interfering channels, wherein theweights calculation and interpolation circuitry generates the weightsfrom the channel estimate for the main channel and the interferencecorrelation matrix, and wherein the weights comprise a weight for eachdata subcarrier for signals received through the two or more antennas.3. The mobile station of claim 2 wherein the preamble symbol istransmitted from the serving base station through the main channel andis received through the two or more antennas, wherein the preamblesymbol comprises substantially time-synchronized transmissions by theserving base station on a predetermined set of subcarriers andinterfering transmissions by the one or more interfering base station onthe predetermined set of subcarriers, and wherein the filterinitialization circuitry is arranged to generate the subcarriermodulation sequences for the main channel and the one or moreinterfering channels for use by the recursive filter in updating thechannel estimates.
 4. The mobile station of claim 3, wherein the weightcalculation and interpolation circuitry calculates an interferencecorrelation matrix, on a symbol-by-symbol basis, from the channelestimates of the interfering channels, and wherein the weightcalculation and interpolation circuitry calculates weights for the pilotsubcarriers, on a symbol-by-symbol basis, from the interferencecorrelation matrix and the channel estimate for the main channel.
 5. Themobile station of claim 3 wherein the filter initialization circuitryinitializes the recursive filter with an initial channel estimate forthe main channel generated by the preamble symbol processing circuitryusing the pilot subcarrier modulation sequences, wherein the filterinitialization circuitry selects orders for the recursive filter basedon a number of interfering base stations identified by the preamblesymbol processing circuitry, and wherein the filter initializationcircuitry adaptively updates the orders of the recursive filter when thenumber of interfering base stations changes.
 6. The mobile station ofclaim 1 wherein the preamble symbol is received through the two or moreantennas, wherein the preamble symbol processing circuitry combinesfrequency-domain signals associated with each of the antennas, andwherein the preamble symbol processing circuitry performsfrequency-domain matched filtering to separate preamble symbolstransmitted by the one or more interfering base stations from a preamblesymbol transmitted by the serving base station.
 7. The mobile station ofclaim 3 wherein the preamble symbol processing circuitry determines apreamble identifier for the serving base station from the preamblesymbol. and a preamble identifier for the one or more interfering basestations, wherein the preamble symbol processing circuitry furtherextracts a seed for the serving base station from the preambleidentifier of the serving base station, wherein the preamble symbolprocessing circuitry further extracts one or more seeds for the one ormore interfering base stations from the preamble identifier of the oneor more interfering base stations, and wherein the filter initializationcircuitry generates the subcarrier modulation sequence for the mainchannel from the seed for the serving base station and generates the oneor more subcarrier modulation sequences for the one or more interferingchannels from the one or more seeds for the one or more interfering basestation.
 8. The mobile station of claim 3 wherein the one or moreinterfering base stations and the serving base station operatesubstantially synchronously baying substantially synchronized OFDMAdownlink time slots, and wherein the preamble symbol is an OFDMAdownlink preamble symbol of an OFDMA frame received by the mobilestation from the serving base station and the one or more interferingbase stations.
 9. The mobile station of claim 3 wherein the weightscalculation and interpolation circuitry comprises: weight calculationcircuitry to calculate the interference correlation matrix from thechannel estimates for the interfering channels and to generatepilot-subcarrier weights for each of the pilot subcarriers from thechannel estimate for the main channel and the interference correlationmatrix; and weights interpolation circuitry to perform a linearinterpolation on the pilot-subcarrier weights to generatedata-subcarrier weights for the data subcarriers for each of the two ormore antennas.
 10. The mobile station of claim 9 further comprising acombiner to apply the data-subcarrier weights to data subcarriersreceived through the two or more antennas to generate one set of symbolsfor the data subcarriers.
 11. A method for reducing interference fromone or more interfering base stations comprising: identifying one ormore interfering base stations from a received preamble symbol;performing a recursive filtering process using pilot subcarriermodulation sequences on pilot subcarrier of orthogonal frequencydivision multiple access (OFDMA) data symbols received through two ormore antennas to update channel estimates on a symbol-by-symbol basisfor a main channel associated with a serving base station and one ormore interfering channels associated with the one or more interferingbase stations for use in mitigating interference from the interferingbase stations, wherein performing the recursive filtering comprisesupdating the channel estimates for the main channel and the one or moreinterfering channels on the symbol-by-symbol basis based on astate-space dynamic model of a channel transfer function for the mainand interfering channels using signals received on the pilot subcarriersand the subcarrier modulation sequences for the main and interferingchannels; generating the pilot subcarrier modulation sequences for themain channel and the one or more interfering channel based on seedsextracted from preamble identifiers transmitted by the serving basestation and the one or more interfering base stations, respectively; andinitializing the recursive filtering process with channel estimatesgenerated from the pilot subcarrier modulation sequences, wherein theserving base station and the one or more interfering base stations usedifferent pilot subcarrier modulation sequences for modulation of theirpilot subcarriers.
 12. method of claim 11 further comprising: generatingweights for subsequent application to each of a plurality of datasubcarriers of the data symbols received through the two or moreantennas; calculating an interference correlation matrix from thechannel estimates for the interfering channels; and generating theweights from the channel estimate for the main channel and theinterference correlation matrix, the weights comprising a weight foreach data subcarrier received through the two or more antennas.
 13. Themethod of claim 12 wherein the preamble symbol is transmitted from theserving base station through the main channel and is received throughthe two or more antennas, wherein the preamble symbol comprisessubstantially time-synchronized transmissions by the serving basestation on a predetermined set of subcarriers and interferingtransmissions by the one or more interfering base station on thepredetermined set of subcarriers, and wherein the method furthercomprises generating the subcarrier modulation sequences for the mainchannel and the one or more interfering channels for use in performingthe recursive filtering to generate and update the channel estimates.14. The method of claim 13 wherein the method further comprises:calculating an interference correlation matrix, on a symbol-by-symbolbasis, from the channel estimates of the interfering channels; andcalculating weights for the pilot subcarriers, on a symbol-by-symbolbasis, from the interference correlation matrix and the channel estimatefor the main channel.
 15. The method of claim 13 further comprising:initializing the recursive filtering process with an initial channelestimate for the main channel generated from the pilot subcarriermodulation sequences; selecting orders for the recursive filteringprocess based on a number of interfering base stations identified; andadaptively updating the orders for the recursive filtering process whenthe number of interfering base stations changes.
 16. The method of claim11 wherein the preamble symbol is received through the two or moreantennas, and wherein the method further comprises: combiningfrequency-domain signals associated with each of the antennas; andperforming frequency-domain matched filtering to separate preamblesymbols transmitted by the one or more interfering base stations from apreamble symbol transmitted by the serving base station.
 17. The methodof claim 13 further comprising: determining a preamble identifier forthe serving base station from the preamble symbol and a preambleidentifier for the one or more interfering base stations; extracting aseed for the serving base station from the preamble identifier of theserving base station; extracting one or more seeds for the one or moreinterfering base stations from the preamble identifier of the one ormore interfering base stations; generating the subcarrier modulationsequence for the main channel from the seed for the serving basestation; and generating the one or more subcarrier modulation sequencesfor the one or more interfering channels from the one or more seeds forthe one or more interfering base station.
 18. The method of claim 13wherein the one or more interfering base stations and the serving basestation operate substantially synchronously having substantiallysynchronized OFDMA downlink time slots, and wherein the preamble symbolis an OFDMA downlink preamble symbol of an OFDMA frame received by themobile station from the serving base station and the one or moreinterfering base stations.
 19. The method of claim 13 wherein theinterference correlation matrix is calculated from the channel estimatesfor the interfering channels and to generate pilot-subcarrier weightsfor each of the pilot subcarriers from the channel estimate for the mainchannel and the interference correlation matrix, and wherein the methodfurther comprises performing a linear interpolation on thepilot-subcarrier weights to generate data-subcarrier weights for thedata subcarriers for each of the two or more antennas.
 20. The method ofclaim 19 further comprising applying the data-subcarrier weights to datasubcarriers received through the two or more antennas to generate oneset of symbols for the data subcarriers.
 21. A non-transitorycomputer-readable storage medium that stores instructions for executionby one or more processors to perform operations for reducinginterference from one or more interfering base stations, the operationscomprising: identifying one or more interfering base stations from areceived preamble symbol; performing a recursive filtering process usingpilot subcarrier modulation sequences on pilot subcarriers of orthogonalfrequency division multiple access (OFDMA) data symbols received throughtwo or more antennas to update channel estimates on a symbol-by-symbolbasis for a main channel associated with a serving base station and oneor more interfering channels associated with the one or more interferingbase stations for use in mitigating interference from the interferingbase stations, wherein performing the recursive filtering comprisesupdating the channel estimates for the main channel and the one or moreinterfering channels on the symbol-by-symbol basis based on astate-space dynamic model of a channel transfer function for the mainand interfering channels using signals received on the pilot subcarriersand the subcarrier modulation sequences for the main and interferingchannels; generating the pilot subcarrier modulation sequences for themain channel and the one or more interfering channels based on seedsextracted from preamble identifiers transmitted by the serving basestation and the one or more interfering base stations, respectively; andinitializing the recursive filtering process with channel estimatesgenerated from the pilot subcarrier modulation sequences, wherein theserving base station and the one or more interfering base stations usedifferent pilot subcarrier modulation sequences for modulation of theirpilot subcarriers.